Field/tissue interactions become pronounced at ultra-high field MRI, causing increased RF inhomogeneity. The magnitudes of the inhomogeneity worsen if the dimensions of the sample coincide with the operating wavelength of the radio frequency fields. However, the advantages of being able to gain improved signal-to-noise ratio (SNR) and spectral resolution nevertheless favour the move towards high field MRI. Hence, to benefit from these advantages, the RF inhomogeneity should be ameliorated. A particular method, active RF shimming, which uses spatially selective pulses [Saekho et al, Magn Reson Med 53(2) pg 479-484, 2005] has been shown to yield significant improvement but there are issues associated with this method, explicitly the use of long duration pulses. This, however, can be circumvented through the use of parallel transmission techniques such as Transmit SENSE [Katscher et al, Magn Reson Med 49(1) pg 144-150, 2003], which require dedicated multi-element RF coils and independently controlled transmit/receive units.
In our previous international patent application number (PCT/AU2006/000311), a focusing scheme is described for a multi-element RF system that can increase the quality of images obtained for local regions of interest. The invention is described with reference to a number of small local coils with particular application to the head and chest.
Multi-element RF coils designed for active parallel transmission can be broadly classified into two categories. Multi-element RF coils which are specifically designed for the transmission only of the spatially selective pulses, while having a secondary receive-only RF coil for the reception of the excited MR signals, or, as a transceive system which can be used for transmission and reception simultaneously. The latter having the advantage of not requiring additional RF coils.
One common design element that all multi-element RF coils share is that some form of mutual decoupling method is employed to decouple the coil elements. A multi-element RF coil usually displays strong mutual coupling between individual coil elements. Some of the undesirable effects include difficulty in tuning and reduced SNR. RF field distortion is also a cause of image artefacts. Hence, minimizing mutual coupling is important.
In our pending international patent application number PCT/AU2008/000245, a counter wound inductor decoupling method is described for minimizing mutual coupling. The invention is described with reference to a number of local coils with particular application to the head. In the new coil structure described herein we preferentially use this method of decoupling, however, other methods of decoupling may be used with the new coil structure. The content of the previous application is incorporated herein by reference.
One other important consideration is the structural design of the coil element itself, the goal being a shape which can produce the highest possible RF field inside a conductive sample. This is crucial for active RF shimming, since ideally sufficient RF energy needs to be presented over the entire region of the sample to successfully excite protons over any region-of-interest (ROI). As the effective penetration depth of the RF field is proportional to the size of the coil element it is sometimes difficult to achieve the desired energy distribution, especially when the ROI is in the centre of the sample.
Irrespective of the applications (e.g. human or pre-clinical imaging) the design principles of a multi-element transmit and/or receive RF system remain the same. With strong spatial constraints, however, conventional coil structures can fail to produce a homogeneous RF field profile and even prove physically infeasible to construct. New designs are required.